1. Field of the Invention
The present invention relates to a transmission line filter, and more particularly to a transmission line filter employed in a high-frequency circuit filter for a high-frequency radio transceiver such as a portable telephone and to a transmission line filter employed in an antenna duplexer.
2. Description of the Related Art
FIG. 1 and FIG. 2 are a schematic exploded perspective view and a perspective view, respectively, of a transmission line filter which has been conceived by the present inventors. In the transmission line filter as shown in FIG. 1, resonators 201 through 203 each of which respectively has one end connected to a ground electrode 701 and respectively constitutes a 1/4 wavelength stripline resonator, are formed on a dielectric layer 101 at predetermined intervals. Further, electrodes 301 through 303 are also formed on the dielectric layer 101. Each of electrodes 301 through 303 has one end electrically connected to the ground electrode 701 and the other end respectively spaced at predetermined intervals from open-circuited end portions of the resonators 201 through 203 so as to be opposed to the resonators 201 through 203.
An input electrode 401 and an output electrode 402 are formed on a dielectric layer 102 which is to be stacked on the dielectric layer 101. The input electrode 401 overlaps the resonator 201 disposed on the input side with the dielectric layer 102 interposed therebetween. The output electrode 402 overlaps the resonator 203 disposed on the output side with the dielectric layer 102 interposed therebetween. A dielectric layer 103, an upper surface of which the ground electrode 701 is to be formed on, is stacked on the dielectric layer 102, and the dielectric layers 101 through 103 are combined into a single unit. Thereafter, the resultant product is fired to form a layered product 1000. As shown in FIG. 2, the ground electrode 701 is formed on the upper and lower surfaces of the layered product 1000 and the side surfaces thereof other than an input terminal portion 601 and an output terminal portion 602. In addition, an input terminal 501, which is insulated from the ground electrode 701 and connected to the input electrode 401, is formed in the input terminal portion 601 formed on one side surface of the layered product 1000. An output terminal 502 which is insulated from the ground electrode 701 and connected to the output electrode 402, is formed in the output terminal portion 602 formed on another side surface of the layered product 1000.
An electrical equivalent circuit of the aforementioned transmission line filter is represented as shown in FIG. 3 and constitutes a bandpass filter. Reference numeral 11 indicates a capacitance between the resonator 201 and the input electrode 401. Reference numeral 12 indicates a capacitance between the resonator 203 and the output electrode 402. Reference numerals 21 through 23 respectively indicate a capacitance between the resonator 201 and the electrode 301, a capacitance between the resonator 202 and the electrode 302 and a capacitance between the resonator 203 and the electrode 303. Reference numeral 31 indicates an inductance induced between the resonator 201 and the resonator 202, and reference numeral 32 indicates an inductance induced between the resonator 202 and the resonator 203. Capacitances 201C, 202C, 203C and inductances 201L, 202L, 203L of parallel resonance circuits respectively correspond to capacitances and inductances obtained by expressing the resonators 201, 202, 203 with lumped constants.
There is a strong demand for a reduction in size of a portable telephone terminal using such a bandpass filter, and there is a strong demand for a reduction in size of the bandpass filter itself used inside the terminal accordingly. However, the transmission line filter having the aforementioned conventional structure has a limit of its size reduction.
Furthermore, in order to use such a bandpass filter as a high-frequency circuit filter for the portable telephone terminal or the like or as a filter for an antenna duplexer, it is necessary to set the bandwidth of the filter to a desired range. However, putting neighboring resonators close to each other to increase the degree of coupling therebetween was the only means to make the bandwidth of the aforementioned conventional bandpass filter broader. However, when the resonators are put so close to each other, characteristics of the resonators become extremely sensitive to variations in manufacturing parameters (e.g., variations in printing parameters) or the like, thereby creating a difficulty in stably supplying a bandpass filter having a constant characteristic.
Moreover, putting neighboring resonators away from each other to reduce the degree of coupling therebetween was the only means to make the bandwidth of the aforementioned conventional bandpass filter narrower. In this case, however, a problem arises that the bandpass filter becomes greater in size when the neighboring resonators are spaced away from each other in this way.